The LTE-Advanced system studied by the 3GPP employs a carrier aggregation (CA) technology (see Non-Patent Literature Document 1) which can concurrently use a plurality of frequency bands (component carriers (CC)) for the purpose of improving frequency usage efficiencies and peak throughputs.
According to the inter-site carrier aggregation (Inter-Site CA) serving as one aspect of carrier aggregation, a mobile terminal device (UE: User Equipment) is concurrently connected to a plurality of base station devices (eNB: E-UTRAN Node B) located at different positions so as to communicate with those base station devices by use of different component carriers (CC).
Additionally, a network configuration has been provided to isolate a C-Plane (Control-Plane) and a U-Plane (User Plane) so as to transmit those planes using different base station devices during communications between base station devices and mobile terminal devices (Non-Patent Literature Document 2).
Herein, macro base stations covering macro cells are base station devices which can be expanded in broad areas in order to secure coverage of mobile terminal devices, wherein those base station devices may use carrier frequency bands such as 800-MHz band. Base station devices covering small cells are base station devices having small coverage, which can be located at various spots to expand capacities, wherein those base station devices may use high-frequency bands such as 3.4-GHz band.
Additionally, PCell (Primary Cell) refers to cells covered by base station devices configured to manage RRC (Radio Resource Control) connections between mobile terminal devices and base station devices according to carrier aggregation, while SCell (Secondary Cell) refers to cells covered by other base stations. Moreover, C-Plane is a protocol used to manage connections with mobile terminal devices while controlling mobility, while U-Plane is a protocol concerning reception and transmission of scheduling information and user traffic.
Moreover, the downlink (DL) indicates a direction of communicating data from a base station device to a mobile terminal device, while the uplink (UL) indicates a direction of communicating data from a mobile terminal device to a base station device.